Purpose

The purpose of this lab is to show how by using continuous variation, one can find the mole ratio of reactants in a reaction.

Data Tables:

Solution

Temperature

Change in Temperature from Average Temperature of Solution B and NaClO

50 mL Solution B0 mL NaClO

21°C

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45 mL Solution B5 mL NaClO

25.5°C

4°C

20 mL NaClO30 mL Solution B

36°C

14.5°C

30 mL NaClO20 mL Solution B

32.5°C

11° C

45 mL NaClO5 mL Solution B

24.5° C

3° C

0 mL Solution B50 mL NaClO

22° C

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Conclusion

By using different amounts of each of the reactants, Solution B and NaClO, changes in temperature were produced.By plotting the data points on the graph and drawing two lines that best fit the data, the point of intersection was determined. The point of intersection on this graph is found at approximately 28 mL of Solution B and approximately 23 mL of NaClO which produces a temperature change of 13.4 degrees Celsius. Both the Solution B and NaClO have a concentration of 0.50M. The ratio of the two volumes of the reactants is the stoichiometric ratio of the reactants, resulting in 0.014 moles of Solution B and 0.0115 moles of NaClO.After changing these coefficients to whole numbers,a stoichiometric ratio of approximately 6 moles of Solution B to 5 moles of NaClO is determined.

Discussion of Theory

In this experiment, the technique of continuous variation was used to determine the molar ratios of reactants. To perform this process, the total volume of the two reactants in the solution must be kept constant. Then, by mixing solutions containing different molar ratios of the two reactants, and measuring a property that is dependent on the concentration of the reactants in the solutions, such as the change in temperature, one can derive the stoichometric ratio of the reactants. If the change in temperature is the property that is being measured, then the maximum change in temperature will take place at the optimal molar ratio. In the experiment that was performed, the volume of the solution with both reactants and the total number of moles of either Solution B or NaClO were kept constant, although the molar ratio between these two reactants was changed.

Analysis Questions

1. Keeping the volume of the reactants constant is necessary to keep the temperature produced proportional the the amount of reactants in the solution and to make the change in temperature comparable to the other tests. Varying the volume of the reactants will alter the data points on the graph and the point of maximum temperature change will not be able to be found, therefore the molar ratio of the reactants will not be able to be determined.

2. The "limiting reagent" is the reactant that is used up completely in the reaction. After the limiting reagent is found, the amount needed of the other reactant to complete the reaction can be found, and the excess amount of the other reactant can also be found.

3. There is error in both the measurement of temperature and volume, but the temperature limits the precision of the data more than volume in this experiment. The temperature of the different solutions is measured with an analog thermometer, and there are many ways in which both random and systemic error can affect the data. If the temperature is measured wrong it will change the line-of-best-fit, therefore changing the stoichiometric ratio of the two reactants.

4. NaClO is the limiting reagent along the upward sloping line of the graph. Solution B is the limiting reagent along the downward sloping line of the graph.

5. The color intensity of the product, the volume of a gas that forms as a product, and the mass of the precipitate that forms, change in pH of a product, turbidity of a product are physical properties that could use the method of continuous variations.

6. It is more accurate to use the point of intersection of the two lines to find the molar ratio rather than the point of greatest temperature change because the point of intersection shows the true point of greatest temperature change although that point was never tested. By using the point of greatest temperature change instead of the point of intersection, all of the different combinations of reactants are eliminated unless they have been tested.When using the point of intersection between the two lines, information gained from the entire experiment is being used, instead of relying on information from one data point.